1. Field of the Invention
The present invention relates to a method and apparatus for high frequency heat-treatment of long continuous metal blanks, particularly, steel pipes of a fixed length to be used for feeding various kinds of fluids or for mechanical structures.
2. Description of the Prior Art
The conventional high frequency heat-treatments of long continuous metal blanks, each having a fixed length, such as metal pipes to be used for brake oil piping for automobiles, piping for feeding liquid or gaseous fuel or other liquids or for mechanical structures, have been carried out by feeding the long continuous metal blanks in their lengthwise direction to pass them through a high frequency heating means and subsequently feed them into a long cooling chamber arranged in alignment with the heating means. In order to feed continuously the long continuous blanks to the conventional heat-treatments a plurality of long continuous metal blanks must be jointed end-to-end to form a single tremendously long metal blank. Therefore, in the conventional heat-treatments it has indispensably been required to provide a means for making the end-to-end joints of the long continuous metal blanks, a means for disconnecting or cutting the end-to-end joints and a means for removing the disconnected or cut end-to-end joints.
However, in view of the use of the high frequency heating means which is capable of effecting a converged heating with a high density and the heating to save the manufacturing cost of the metal pipes the conventional heat-treatments have been required to feed the metal pipes of fixed length to be heated to the high frequency heating means at a high speed and, prior to subjecting to the high frequency heating means, to carry out the end-to-end jointing step of the metal pipes by heating them and using a jointing means at a speed synchronized with the metal pipe feeding speed.
Furthermore, according to the conventional heat-treatments it is required to provide a very long cooling chamber for cooling the tremendously long metal pipe which was passed through the high frequency heating means which effects a rapid heating. (Usually, the cooling chamber must be formed about 3 to 8 times longer than the long length of metal pipe, though the length of the cooling chamber is varied with the diameter or wall thickness of the metal pipes, the magnitude of high frequency energy or the pipe feeding speed.) If the length of the cooling chamber is shorter than that as above defined, the metal pipe will be insufficiently cooled so that the surface thereof may be oxidized after discharged from the cooling chamber. The provision of the long cooling chamber makes inevitably the installation of the apparatus so large as to occupy a space of more than several ten meters for feeding the tremendously long continuous metal blanks to be heated. This occupation of the space will be further enlarged because of the addition of the end-to-end jointing means, disconnecting or cutting means and removing means to the front of the heating means or the back of the cooling means in series. Such enlarged occupation of the space issues often a problem in the lay-out of the plant.
In addition, according to the conventional heat treatments the tremendously long metal pipe which was discharged from the cooling chamber must be immediately subjected to the disconnection or cutting of the joint portions thereof o removal of the joint portions in synchronization with the pipe feeding speed so that the production efficiency may be reduced and the cutting or removal of the joint portions may be resulted in a production of scraps, thereby deteriorating the yield of production and causing the production cost to be expensive.